A carbon nanostructure of the present invention is a nano-sized substance composed of carbon atoms. Examples of the carbon nanostructure include carbon nanotubes, carbon nanotubes with beads, brush-type carbon nanotubes that are a forest of carbon nanotubes, carbon nanotwists that are carbon nanotubes having twists, carbon nanocoils in coil form, and fullerenes in spherical shell form. In the following description, these many carbon substances are collectively referred to as carbon nanostructures.
As a method for producing these carbon nanostructures, known are: chemical vapor deposition (CVD) method of decomposing raw material gas, such as hydrocarbon, so as to form a target substance; and catalyst chemical vapor deposition (CCVD) method of forming a target substance by catalysis. The CCVD method is merely one form of the CVD method.
The present invention relates to a method for producing carbon nanostructure by the foregoing CVD method. The CVD method collectively means a method of decomposing raw material gas in a reaction container so as to cause a catalyst to form a target substance. Decomposing means includes various kinds of decomposing means such as heat, electron beam, laser beam, and ion beam.
Conventionally, in order to produce carbon nanostructure by the CVD method, adopted is a method of introducing a mixed gas of raw material gas and carrier gas into a reaction chamber and decomposing raw material gas by catalysis so as to form carbon nanostructure on a catalyst.
FIG. 18 is a schematic block diagram of a carbon nanostructure synthesis apparatus 50 that is a first conventional art. The conventional carbon nanostructure synthesis apparatus 50 is arranged such that a catalyst support 6 is placed in a reaction chamber 4, and carrier gas and raw material gas are fed in the reaction chamber 4 so that carbon nanostructure 8 grows on the surface of the catalyst support 6.
Carrier gas, such as He, delivered from a carrier gas container 10 is decompressed by a regulator 12, controlled to be of a predetermined flow quantity by a carrier gas flow quantity controller 14 such as a mass flow controller, and then fed at a necessary timing from a carrier gas valve 16.
Meanwhile, raw material gas, such as C2H2, delivered from a raw material gas container 18 is decompressed by a regulator 20, controlled to be of a predetermined flow quantity by a raw material gas flow quantity controller 22 such as a mass flow controller, and then fed at a necessary timing from a raw material gas valve 16.
Carrier gas and raw material gas are mixed in a merge section 32, and jetted through a gas feed nozzle 34 into the reaction chamber 4 in a direction indicated by an arrow c. Raw material gas is decomposed by a catalyst support 6. On the surface of the catalyst support 6, the carbon nanostructure 8 is synthesized, and unwanted gas is exhausted from a delivery pipe 36 in a direction indicated by an arrow d.
FIG. 19 is a time series graph showing initial instability of flow quantity of raw material gas in a raw material gas flow quantity controller 22 of the conventional apparatus. As the raw material gas flow quantity controller 22, a mass flow controller is usually used, and the mass flow controller is designed so as to regulate a flow quantity of raw material gas to a predetermined flow quantity under PID control.
The PID control is a control scheme realized by a combination of proportional control, integral control, and differential control. It is considered that the PID control is the most presently excellent control scheme. This control scheme needs several seconds to control the flow quantity of raw material gas to a predetermined flow quantity. In the initial stage of the process, the flow quantity repeats overshoots and undershoots and finally reaches the predetermined flow quantity. This initial fluctuation of the flow quantity is inevitable. Initial fluctuation time ΔT reaches several seconds.
According to a research conducted by the inventors of the present application, it becomes clear that initial fluctuation of the raw material gas flow quantity had a significant influence on the initial growth of the carbon nanostructure 8. The carbon nanostructure 8 is nano-sized. Accordingly, a growth time of the carbon nanostructure 8 is extremely short. Under synthesis conditions where growth of the carbon nanostructure 8 is stopped in several seconds, the initial fluctuation of the raw material gas flow quantity has adverse effect of synthesizing carbon nanostructures. In view of this, a way of preventing the initial fluctuation of the raw material gas was devised.
FIG. 20 is a schematic block diagram of a carbon nanostructure synthesis apparatus 50 which is a second conventional art. A configuration of the carbon nanostructure synthesis apparatus 50 is the same as that of the carbon nanostructure synthesis apparatus 50 in FIG. 18 except that the raw material gas valve 52 is replaced with a manual three-way valve 54.
In this conventional art, the manual three-way valve 54 is used for the purpose of avoiding the initial fluctuation of the raw material gas. The raw material gas flow quantity controller 22 controls a flow quantity of the raw material gas to a predetermined flow quantity. In an interruption state of the raw material gas of this predetermined flow quantity, the raw material gas is exhausted to a supplemental exhaust pipe 54b in a direction indicated by an arrow a.
To feed the raw material gas to the reaction chamber 4, the manual three-way valve 54 is switched by manual operation, and the raw material gas of a predetermined flow quantity is fed to an injection pipe 54a in a direction indicated by an arrow b. With this arrangement, effects of initial fluctuation time ΔT under PID control can be avoided. However, another problem arises.
FIG. 21 is a time series graph showing slowness of opening/closing the conventional manual three-way valve for a raw material gas flow quantity. Manually switching the manual three-way valve 54 takes time. In feeding raw material gas, an opening operation for switching from the direction indicated by the arrow a to the direction indicated by the arrow b occurs a rising time ΔT1. Similarly, in interrupting raw material gas, a closing operation for switching from the direction indicated by the arrow b to the direction indicated by the arrow a occurs a fall time ΔT2.
Especially, the rising time ΔT1 causes instability of flow quantity in an initial stage, which is a factor adversely affecting initial growth stage of the carbon nanostructure 8. When the growth time of the carbon nanostructure 8 is long, the fall time ΔT2 does not have so large influence on the growth of the carbon nanostructure 8, but the rising time ΔT1 has some influence on the growth of the carbon nanostructure 8.
As is apparent from the first conventional art and the second conventional art, in initiating feed of raw material gas, existence of the initial fluctuation time ΔT and the rising time T1 of the raw material gas flow quantity adversely affects initial growth of carbon nanostructures. In addition, distortion of the initial growth might inhibit the flowing growth even after some time.
Therefore, the present invention provides method and apparatus for high-efficiency synthesis of carbon nanostructure which eliminates the initial fluctuation time and rising time of raw material gas flow quantity at the feed of raw material gas, and instantaneously initiates the contact between raw material gas and a catalyst under the reaction conditions so as not to adversely affect the initial growth of the carbon nanostructures, and an object of the present invention is to produce a high-purity carbon nantostructure.